Process of removing scale.



FRANK E. COOMBS, OF AU SABLE FORKS, NEW YORK.

PROCESS OF REMOVING SCALE.

No Drawing,

Specification of Letters Patent. Application filed July 8, 1911. Serial No, 637,440.

Patented Dec. 31, 1912.

rators, vacuum pans, grainers, heaters, etc.,

which are advantageously first cleansed with water or the like,are treated with a solution of caustic soda at a temperature above 100 C. (212 F.) to produce a disintegration of the scale, such solution being advantageously filmed or otherwise distributed over the scale, and the scale is thereafter removed, the soda being, if desired, regained for subsequent use; all as more fully hereinafter set forth and as claimed.

In the operation of many heat-transferring liquid-treating devices, such as evaporating pans, vacuum pans, boilers, grainers, heaters, etc., the surface exposed to the liquid after a time scales'up, much impeding transference of heat.

as the carbonate, sulfate, sulfite, citrate, aconitate, etc., the particular acid or acids combined with thecalcium varying with the liquid treated; and it often also contains various mechanical and other impurities such as mud, iron salts, alumina compounds, silica and silicates, sticks, straws, sand, etc. With boilers, ordinarily the scale is largely carbonate or sulfate; with waste sulfite liquors and the like it may be largely sulfite; with sugar juices it may be all these salts and also calcium salts of organic acids, etc.

The scale is often firmly adherent and as in modern heat transferring devices, the heating surfaces are relatively large and are hard to get at, its removal isdifiicult. This is particularly the case where, as in many types of evaporators, the heating and evaporating of the liquid takes place from the external surface of tubes of relatively small diameter- Scale forming on the outside of such tubes is difiicult to remove, owing to the lack of room between the closely set tubes and the, consequent impracticability of reaching their external surfaces with scraping tools.

'In the prior art many attempts have been,

The scale is usually composed of or comprises calcium salts, such made to remove scale with one solution or another, but without any great degree of success.. Its removal is usually an expensive and tedious operation, often requiring the apparatus to be shut down for a long'period of t1me' and to be more or less dismantled. With any amount of scale on the heat transferring surfaces the efficiency goes down very markedly. Often mechanical means of one type or another, such as scrapers, brushes or the like are employed in removmg scale but it is difficult to prevent mechanical injury thereby and particularly where the heat transferring element is, as is frequently the case, of copper; copper being a relatively soft metal.

In the present'invention I have devised a .simple, cheap, ready and rapid method of solubilizing or disintegrating and removing scale from evaporating surfaces, based on the fact of a reversal of the action of soda upon lime or calcium salts occurring when a caustic soda solution is above 1.10 sp. g. and 100 C. (212 F At most ordinary densities and temperatures the action of lime upon a soda salt of'an acid forming an insoluble calciumsalt is to form such a salt and to set free caustic soda. For example,

quicklime with a solution of sodium carbonate forms calcium carbonate and free caustic soda; this being the ordinary method of making caustic soda from carbonate. With sodium sulfate, the reaction, though not so ready, tends to go in the same direction. If. however, insoluble calcium compounds, such as the carbonates or sulfates be treated with a caustic soda solution 'ata temperature above 100 C. and with a relatively strong soda solution, the reaction goes in the reverse direction, the calcium salts being converted into more or less soluble sodium salts. The strength of the soda solution in contact with the scaled surface should be at least 12 to 13 B. (l.O91-l.10 specific gravity) and is best about 30 B. or higher. With a weak soda solution it is particularly advantageous to perform the scale-disintegrating action at Y a temperature above the normal boiling point of the liquid, the liquid being heated under pressure. Caustic soda solutions of less than about 1.10 s ecific gravity even at their boiling points a cot many scale-forming materials to only a slight degree. With a stronger hot soda solution, scale removal may be performed at the ordinary atmospheric pressure. It is also best to present the soda solution to the surface in arelatively subdivided form as by spraying-or traversing it along the scaled surface ma relatively thin film or layer. With modern filming evaporators or vacuum pans this subdivision offers no difiiculties. The same is trueof many of the flash types of boilers or evaporators.

Before treatment with the soda, the scaled surface is preferably thoroughlycleansed by treatment with water or steam to remove soluble bodies, since the solution of these soluble bodies in the soda lye, which is usually to be recovered and used again, is disadvantageous. After the treatment the scale will be disintegrated and may be removed with acids or mechanically, as by forcible impingement of jets of water against" the incrusted surface, or by the usualforms of stiff brushes or scrapers, or by other chemical or mechanical means, or by both chemical and mechanical means.

I The action of the soda is to convert the lime salts into new forms, the corresponding contact between soda an sure.

acids of the original lime salts dissolving in I the soda to a greater or less extent and this action is accompanied by disinte 'ration or loosening ofthe scale. And the hlgher temperatures have the further merit of more rapidly attacking many. organic bodies which are present, and which act as a cement among the various layers of the scale, which has always a laminated structure. After the soda treatment, the scaled surface should of course be washed to recover any adhering soda. Very frequently this washing operation alone suifices to remove the scale. The washing operation is best done with hot water.

The means to be emplo ed for producing scale during'the disintegration treatment may be as desired and'will vary with the articular apparatus to be treated. As stated: it is best to use the soda solution in subdivided form, as this ives a better action, but the scaled surface may be submerged in the soda if desired. This however involves the use of relatively large amounts of soda; neither is the action so quick. In the caseof afilm evaporator, the soda solution may be simply caused to follow the usual path of liquid through the apparatus to be treated. As film evaporators are ordinarily constructed to work under vacuum, '11. e. at a relatively low heat, it is necessary in cleaning them to use either a strong soda solution, as one of 25 B., or witha weaker one of about 13 B, to arrange the valves, etc., so as to. allow performing the treatment under some pres- With other types of apparatus, the soda solution may be sprayed or-film'ed over the heat transferring surface by any appropriate means.

I have found that this method is parmeantiticularly applicable to the treatment of scaled evaporating elements of the climbing film type of evaporator. In these evaporators, evaporation .is performed interiorly of relatively long and narrow tubes, the interior of which is rather inaccessible; and Wheresuch evaporators are employed for treating rapidly scaling liquids, such as waste sulfite liquor, the use of the present invention is particularly advantageous. In modern practice (see Patent 833,634) sulfite waste liquor comingfrom paper pulp digesters is carefully neutralized with lime or 'dolomitic lime and evaporated to a thick liquor of about 30 B. As the liquor'contains some sulfitesand sulfates and some the soda is circulated through the apparatus cyclically and where evaporation is allowed to take place in the apparatus, it-may be necessary to add water to the system at some point to replace evaporation losses and preserve the desired density. After treatment of the scale from the apparatus, the soda is contaminated by dissolved salts of the acids removed from the scale, such as sulfates, sulfites, carbonates, etc, and in order to regain the soda in active form, the solution must be suitably treated. The best method of doin this is to add Water to re duce the speci c gravity below 1.10 and then treat with quicklime. The reduction of specific gravity causes precipitation of lime salts insoluble in caustic soda solutions of less than 1.10 specific gravity, in so far as such salts or their acids have been dissolved, with accompanying regeneration of part of the caustic soda. The addition of quicklime completes the precipitation of insoluble lime salts and recovers all the remaininggcaustic soda. The lime is best used as quic lime since the heat of hydration materially assists in causing the desired reactions. The regenerated soda solution so produced is too dilute for the present purposes but it may be readily brought up to The caustic soda solution may be passed ioo the density desired by allowingl evaporation 4 to the desired extent during t solution in the scale removing operation.

After cleansing an evaporator with soda solution, the latter may be treated with lime in the described manner and the mixture allowed to stand until the solution is again required; The solution may thenbe drawn e use of the roaaoea 53 supplying Water to the caustic solution at any convenient point in its circulation, as by means of a ba1l cock or float valve.

\Vhile I have described more particularly the use of a solution of caustic soda, re-

A or spraying it thereover.

garding such caustic soda solution as particularly advantageous for the present ur-' poses, yet-I may use a solution of sodium carbonate under heat and pressure. With a hot solution of sodium carbonate under pressure the disintegrating action upon scale is much more pronounced than where such solution is used under less pressure or at a less temperature. I may therefore use carbonate of soda circulated through the evaporating apparatus to be cleansed in the same manner as caustic soda, using a strong solution and finely dividing it over the scale surfaces, as by fihning it thereon In cleansing vacuum apparatus with a sodium carbonate solution, the solution may be caused to follow the normal course of liquid to be evaporated through the ap aratus, but the apparatus should prefera ly, instead of being worked under vacuum, be worked under .ressure. It is best to use a sodium caronate solution relatively strong and to cause the heat to be above the normal boiling oint of the solution used under at.- mosp eric pressure.

While caustic potash and potassium carbonate the be em loyed in lieu of caustic soda or so ium car onate, they are more expensive and ordinarily offer no special advantage over soda. As, however, the solutions may be regained, the item of additional expense is not very great.

What I claim is 1. In removing scale from evaporating elements, the process which comprises subjecting the scaled evaporative surfaces of such elements to the action of a heated solution of caustic soda at a temperature above C. and of a density above 1.10 specific gravity.

- 2." In removing scale from evaporating elements, the process which comprises subje'cting the scaled evaporative surfaces of nesses.

such elementsto the actionof a heated solution of caustic soda of a density exceeding 1.10 specific gravity.

3. In removing scale from evaporative surfaces, the process which comprises washing the scaled surfaces to remove soluble matters, subjecting the washed surfaces to the action of a heatedsolution of caustic soda at a temperature above 100 0., and Q thereafter removing the softened scale.

4. In removing scale from evaporative surfaces, the process which comprises treating the scaled surfaces with water to remove soluble matters, subjecting the washed surfaces to the action of a heated solution of caustic soda of a density exceeding 1.10 specific gravity and thereafter re moving the softened scale.

5. In removing scale from evaporative surfaces, the process which comprises water treatment of the surfaces to remove soluble matters, subjecting the washed surfaces to the action of a heated solution of caustic soda of a density exceeding 1.10 specific gravity and at a temperature above 100 0., and thereafter removing the softened scale.

6. In removing scale from evaporating surfaces, the process which comprises treating such surfaces with a traveling film of alkali solution at a density above 1.10"

specific gravity.

7. Inremoving scale from evaporating surfaces, the rocess which comprises treating such sur aces with a traveling film of caustic soda solution of not less than about 1.10 specific gravity and at a temperature above 100 C.

8. In removing scale from evaporating surfaces, the process which'comprises treatin such surfaces with a solution of caustic.

"with said surfaces to a temperature above the boiling point of saidsolution.

10. In removing scale from evaporating surfaces, the process which comprises circulating a heated solution of a caustic alkali in relatively thin layers over such surfaces.

In testimony whereof, I aflixmy signature in the presence of two subscr'ibingmt- FRANK n. .oooMBs. Witnesses L. O. Beams, Jae. Snmms. 

